1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device having a shallow trench isolation (STI) structure and a manufacturing method thereof, and more particularly to a nonvolatile semiconductor memory device in which an insulating film having fluidity is used as an STI filling material, and a manufacturing method thereof.
2. Description of the Related Art
Recently, flash memories are increasingly scaled down and realized higher bit density. Due to the rapid scale-down of the device, it becomes also important to shrink cell transistors and shrink element isolation regions that occupy most of the element area.
As an element isolation region forming method, an STI technique for filling insulating films into trenches formed by anisotropic etching is used. With the present STI technique, a filling process is performed by use of silicon oxide films formed by a high density plasma (HDP) assisted-chemical vapor deposition (CVD) method. However, it becomes more difficult to perform the filling process by use of the silicon oxide films by the HDP-CVD method as the miniaturization is further advanced. This is because openings are more frequently closed by formation of overhang films when the STI width becomes less than 30 nm although the HDP-CVD method is originally a highly anisotropic film formation method.
Therefore, recently, use of an insulating film such as a spin on glass (SOG) film, TEOS/O3 film or chemical vapor condensation film having fluidity at the filling time of or at the heat treatment time as the STI filling material is studied (see Jpn. Pat. Appln. KOKAI Publication No. 2001-319927, Japanese Patent Publication No. 3178412).
However, since the above insulating films having fluidity generally have low film density and exhibit large film condensation, they tend to cause high tensile stress in an element region. Therefore, there occurs a problem that occurrence of crystal defects and deformation of the element region may be caused due to the stress applied to the active area.